The present invention relates to the fabrication of conductor thin films which are essential to any practical application of superconductivity to microelectronics. The invention permits the production of superconductor oxide films at temperatures significantly lower than have been possible before.
High temperature superconductors may be especially useful when formed into thin films for use in computers and integrated circuits. There has been some success in this work. Researchers have made very high quality thin films out of not only the Y-Ba-Cu-O compound but also the recently discovered thallium-based superconductors. The problem is that the films had been made under conditions that would not transfer well to semiconductor manufacturing. There have been numerous reports of techniques for growing thin films of the new oxide superconductors. These reports demonstrate not only the tremendous interest and importance of these films, but also the relative ease with which they can be synthesized, as compared with the previous "high-T.sub.c " superconductors, namely the A-15's. The new films have been grown by e-beam co-evaporation.sup.1-4 and sequential evaporation,.sup.5 sputtering,.sup.6-9 ion-beam deposition,.sup.10 molecular beam epitaxy,.sup.11-12 laser ablation,.sup.13 spray.sup.14 and spin-on.sup.15 techniques.
To integrate superconducting thin films into existing semiconductor technology, the films need to be placed on silicon--the basic material on which almost the entire semiconductor industry is based. But the techniques for making superconducting films, while successful with specialized base layers or substrates, were unusable in the presence of silicon. The problem was that the temperatures used in the processing were too high. Only one group has reported a deposition temperature of 450.degree. C. with no post anneal.sup.9. However, the substrates used by this group were immersed in a strong plasma discharge which makes it likely that the substrates were actually at a higher temperature.
The processing techniques for making superconducting thin films vary, but they have certain steps in common. The substrates must be heated and bombarded with atoms or molecules of the materials that will make up the superconducting layer. Then once the thin film condenses onto the substrates, the material generally needs an oxygen treatment at 850.degree. C. or higher to transform it from a non-superconducting form to one that is superconducting. Unfortunately, if this is done on a silicon substrate, the high temperatures in the processing cause the superconductor to react with the silicon molecules, changing the composition of the film to destroy its superconductivity.
The low process temperature is desirable not only to avoid the problems already mentioned by Wu et al.,.sup.13 but also to allow the growth of good films on practical substrates (i.e., other than SrTiO.sub.3), or substrates with semiconductor devices. For scientific applications, the capability of using different substrates is important since in many measurements (e.g., optical and radio frequency), the properties of the substrate may obscure those of the film. Most vacuum thin film work to date involves the use of existing vacuum systems with only minor modifications. Large oxygen pressures (up to 10.sup.-3 Torr) have been used in vacuum chambers designed to operate at much lower pressures. The rates become irreproducible with high oxygen pressures because the quartz crystal rate monitors measure the mass deposited on them and therefore the measured rate depends on the degree of oxidation of the metal deposited on the monitor. Rate monitors of other types do not operate at all at such high pressures. Another potential problem due to high pressure is the scattering of the evaporant with the background gas which not only alters the rates but may also produce significant cross talk between different rate monitors. Finally, high pressure can cause e-beam sources to arc or have a severely reduced filament lifetime. The system of the present invention allows the operation of the sources and rate monitors at low pressure while the substrate receives a high flux of oxygen from a plasma source.
Until now, the only good superconducting thin films were made on the substrates such as strontium titanate, that did not react with the superconducting film at such temperatures. The present invention involves a process and apparatus that allow the synthesis of good quality films at temperatures below 550.degree. C. This process yields the highest transition temperatures reported for films on silicon substrates.